public class linked_list_cycle {
    //快慢指针
    public boolean hasCycle(ListNode head) {
        if(head==null||head.next==null) return false;

        ListNode slow=head;
        ListNode fast=head.next;
        //如果有环，一定会相遇
        while(fast!=slow){
            if(fast==null||fast.next==null) return false;
            slow=slow.next;
            fast=fast.next.next;
        }
        return true;
    }
}
